Generally, due to mechanical limits, a polishing machine is designed smaller than a large scale plate. In this case, in order to polish the entire surface of the large scale plate, the polishing machine moves while polishing the substrate.
Referring to FIG. 1, a travel range P1 by which an upper plate 1 of a general polishing machine moves in the longitudinal direction (x direction) of a substrate 2 is 60% to 80% of the radius of the upper plate 1, and a travel range P2 by which the upper plate 1 moves in the transverse direction (y direction) of the substrate 2 is 50% to 70% of the radius of the upper plate 1. In FIG. 1, a rectangle depicted with dots represents a path along which the center of the upper plate 1 moves during a polishing process.
If the upper plate 1 polishes the large scale plate 2 while moving as much as the travel ranges P1 and P2, the center portion of the large scale plate 2 is more polished, but the rim portion of the large scale plate 2 is less polished, which cause a serious polishing deviation. FIG. 2 shows the polishing deviation, in which a portion with a more polishing amount is depicted in red and a portion with a less polishing amount is depicted in blue.
The polishing deviation is generated since, when the upper plate 1 moves along the path 10, the upper plate 1 always passes over the entire center portion but passes over a part of the rim portion which corresponds to just ¼ of the center portion.
Meanwhile, the polishing amount of a polishing pad is influenced by the wear condition of the polishing pad and the pressure applied to the substrate 2 by the polishing pad. Therefore, in order to decrease the polishing deviation, the wear condition of the polishing pad and the pressure applied to the substrate 2 by the polishing pad should be considered together.